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checking Brunswick

@Edano Exactly! And yet the NRC protocol now seems to insist that all BWR Mark I plants installed the modification....

A second later I find it : ajw.asahi.com Thanks for posting it Edano!

I found this in the latest NRC staff recommendation based on the earlier Fukushima Task Force Report, which I cannot find right now. Regardless here find an excerpt from the recommendation bit.ly : "Staff Assessment and Basis for Prioritization BWR Mark I primary containments should have a reliable hardened vent for mitigating beyond-design-basis events. The NTTF recommendation aligns with long-standing staffrecommendations for the Mark I as documented in SECY 89-17, I found this NRC staff recommendation based on the Fukushima Task Force Report. I cite the pertinent part “Mark I Containment Performance Improvement Program,” and GL 89-16. The Fukushima Daiichi accident highlighted the importance of the wetwell vent function, the accessibility of the valves and the capability for operation independent of AC power. All Mark I plants have installed a hardened vent. The degree to which the vent can be used during an extended SBO relies on actions taken to comply with 10 CFR 50.54(hh)(2). Further, these vents have not been designed to any standard governing “ease of use,” comprising accessibility and operability under a range of conditions including SBO and high radiation fields. The staff has also performed extensive studies of the safety benefits of containment venting for the Mark II containment design, which like the Mark I is an inerted pressure suppression design,although of larger volume. The Mark II was not included in the scope of GL 89-16 at the time it was written in part because the wetwell scrubbing of a fission product release from containment resulting from a beyond-design-basis accident was felt to be less certain than for the Mark I. In light of Fukushima Daiichi and the benefits of preserving the integrity of the containment in beyond-design-basis accidents, Mark II plants should have a reliable hardened vent either on the basis of either (1) more recent analysis that acceptably reduces the uncertainty of a scrubbed release, or (2) a design that reduces the uncertainty.The staff concludes that it would be appropriate to redefine what level of protection of public health and safety should be regarded as adequate for venting of BWR Mark I and Mark II primary containments. In addition, the issue of containment vent filtration has been raised as an additional recommendation which is undergoing further staff review. The staff will need to develop a regulatory basis and acceptance criteria for the reliable hardened vent that encompasses prolonged SBO operation, vent capacity, accessibility, and safe hydrogen and fission product processing for both Mark I and Mark II containments.The staff concludes that this recommendation would improve safety. Since sufficient resourceflexibility, including availability of critical skill sets, exists, the staff prioritized this action as aTier 1 recommendation."

I believe the one without any hardened vent is a Mark II. Dresden 1 is retired. You may see the problems in my quotation though. If a station blackout like in Fukushima happened in this country today, the operators would be searching with flashlights for the valves in the reactor building like tepco did. Hope that helps.

@Peter I must admit that I haven't followed your research as closely as I should have, I know that you've been following this thread for quite some time. Thanks for your input!

@smoss , I found my memory back and another nugget. It is not that NRC has not known the weaknesses for a long time: "...with the existing hardware and procedures at some plants, it may not be possible to open or close the vent valves for some severe accident scenarios.
The staff concluded that venting, if properly implemented, could have a significant benefit on plant risk. However, venting via a sheet metal ductwork path, as currently implemented at some MARK I plants, would be likely to greatly hamper or complicate post-accident recovery activities, and was therefore viewed by the staff as yielding reduced improvements in safety. The capability to vent has been recognized as important in reducing risk from operation of MARK I plants for loss of long-term decay heat removal (TW) events, provided the potential downsides of using existing hardware are corrected. Controlled venting can prevent the failure of ECCS pumps from both inadequate NPSH and re-closure of the ADS valves." This quotation is taken from this report issued in 1992: www.nrc.gov

@smoss , nothing but well-meaning suggestions with conditionals throughout. Nobody knows whether the diverse designs would work adequately, if needed. What also bugs me is that the effluent is released unfiltered, except for what is left behind in the suppression pool.